It is well known that patterns of light may be formed by directing light rays through a mask toward a flat surface. This masking method may also be used to selectively remove a photosensitive material from a flat surface: the photosensitive material is applied to the flat surface; the mask is placed on top of the photosensitive material; light is directed through the mask to the photosensitive material; and then either the exposed or unexposed photosensitive material is removed, depending upon whether a "positive" or "negative" type of photosensitive material is used.
A mask may also be used to expose to light selected portions of a surface having upper and lower portions. However, if one seeks to expose only the upper portions of the substrate surface, it becomes necessary first to create a mask which will achieve this effect and then to properly align the mask with respect to the substrate surface to be exposed. The disadvantage of this approach is that as the distances between the raised portions of the substrate surface become increasing in small (e.g. on the order of 100 .mu.m), the problem of aligning the mask and substrate surface becomes increasingly large.
U.S. Pat. No. 4,670,095 to Negishi discloses a maskless method for selectively exposing the sides of a sawtoothed-ridged surface (such as a fresnel plate). The patent teaches that a photosensitive agent may be selectively removed from the sides of the sawtoothed ridges by projecting light rays toward the photosensitive-covered surface at an oblique angle so that the sides of the ridges which face the light are exposed, while the opposite sides of the ridges are not exposed. While this method may be useful for removing photosensitive material from the sides of sawtoothed-shaped ridges, the method is less effective at removing photosensitive material from the tops of rectangular-shaped ridges (i.e. ridges having substantially vertical walls and a horizontal top) because the oblique light would not be normal to the horizontal surface of the tops of the rectangular ridges, thus decreasing the efficiency of the exposure, as well as exposing the sides of the ridges which face the light source, which may be undesirable. Furthermore, this "shadow" method becomes increasingly less effective as the distance between the ridges increases and the height of the ridges decreases, malting it difficult to expose the tops of the ridges without also exposing the lower surface of the substrate between the ridges. It would be desirable to have an effective method for exposing the raised portions of a substrate surface without exposing the lower portions of the surface, even where the height of the raised portions is small compared with the distance between them.